The solubility and diffusion of molecular oxygen in a biological membrane are important variables for understanding respiratory function and dysfunction. The permeability of a membrane to oxygen is related to these parameters and to the magnitude of a proposed interfacial resistance to oxygen transport across a membrane. In this proposal we show that the ESR linewidths of spin probes are sensitive to the product of the concentration and diffusion coefficient of oxygen in a membrane via the spin-exchange mechanism. Using site specific spin probes we have obtained an oxygen concentration-diffusion profile across a bilayer membrane. The effects of phospholipid composition, cholesterol composition, and of temperature will be explored. A time dependent ESR experiment is proposed that will measure the oxygen permeability across hydrated multibilayer lipid films. Work to date suggests that the phospholipid head group region represents a significant resistance to oxygen transport. Chemical titration of dissolved oxygen in aqueous bilayer dispersions will give an ESR independent measure of oxygen solubility in bilayers. Experiments with a high resolution spin probe have yielded signals from multiple sites in bilayers composed of lecithin and cholesterol. These multiple sites will be explored further using high frequency (Q-band) ESR. Oxygen in real membranes, e.g., erythrocytes, will be studied with spin probes.